The present invention is related to bleed air systems employed on aircraft, and in particular to the control of valves employed in bleed air systems.
Bleed air in gas turbine engines is compressed air taken, typically, from the compressor stage of the engine. Bleed air is utilized in a variety of applications on the aircraft, including de-icing operations, as pneumatic power for actuators, and as an input to environmental control systems (ECS) used to provide conditioned air to the cabin.
Control valves are utilized throughout the aircraft to control the flow of bleed air to various loads. Pressure and/or flow sensors monitor the pressure/flow of bleed air provided to various loads and provide the monitored value(s) as feedback to a controller that selectively opens/closes one or more valves to regulate the pressure and/or flow of bleed air.
If the flow/pressure of bleed air provided by the engine is too low (e.g., when the aircraft engine is idling on the ground), the valves being controlled by the controller may become saturated. The term “saturated” is used herein to refer to a condition in which a valve is fully opened, but feedback indicates that the valve should be opened further to increase the flow/pressure of bleed air. Because the valve cannot be opened any further, the valve is said to be saturated and control algorithms used to regulate the pressure/flow of bleed air cannot provided further regulation. That is, the controller is no longer in control of the flow of bleed air. A bleed flow transient (caused for example, by an increase in engine thrust) combined with the saturated position of the flow control valve may result in undesirable transients for loads connected to receive bleed air. For example, such an event may lead to a transient overspeed condition (i.e., speed excursion) for an air cycle machine (ACM) employed as part of the ECS. Over time, these transient events can degrade the health of the attached loads such as the ACM.